Marine Biology

, Volume 127, Issue 1, pp 143–150 | Cite as

Habitat structure, population abundance and the opportunity for selection on body weight in the amphipodEogammarus oclairi

  • O. Iribarne


The importance of oyster-shell habitat-characteristics (depth of the shell layer or degree of fragmentation) to the amphipodEogammarus oclairi were studied in Grays Harbor estuary (Washington, USA) in regard to their effect on local density ofE. oclairi, the risk of predation by juvenile Dungeness crabsCancer magister, and the opportunity for selection on body weight during pairing.E. oclairi was the most abundant macrofaunal species in the intertidal oyster-shell assemblages (density range = 20 to 8500 amphipods m−2), and its density was positively correlated with the depth of the oyster-shell layer (r = 0.85,n = 30,p < 0.005). Field experiments showed that amphipod density was much lower when oyster shells were whole (x = 340.7 ± 72.1,n = 10) as opposed to fragmented [x = 41.4 ± 9.3,n = 10;t c (Welch's approximatet-test) = 13.0,df = 9.3,p < 0.05]. Densities of a predator, juvenileC. magister, were not affected by depth of the shell layer nor by degree of fragmentation (whole shell,x = 14.6 ± 13.0 crab;fragmented shell,x = 18.7 ± 2.4 crab;t c = 0.83,df = 9.7,p > 0.05). Predation rate on single amphipods by crabs did not differ between habitat types (whole shells vs fragmented shells), but mating pairs were consumed more often in the whole-shell treatment (whole shell,x = 0.9 ± 0.8, fragmented shell,x = 0.1 ± 0.3). There were no differences in the size of single amphipods (both sexes) consumed between treatments, or in the size of the paired males consumed. Field experiments showed that the opportunity for selection (i) on male body weight increased with increasing amphipod density which, in turn, increased with increasing degree of shell fragmentation (whole-shell treatment,i = 0.0014 ± 0.0002, fragmented-shell treatment,i = 0.3756 ± 0.0338). Large spatial and temporal fluctuations in population abundance complicate the evaluation of the importance of selection in determining traits such as body weight.


Body Weight Field Experiment Habitat Type Temporal Fluctuation Predation Rate 
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Copyright information

© Springer-Verlag 1996

Authors and Affiliations

  • O. Iribarne
    • 1
  1. 1.Departamento de Biología (FCEyN)Universidad National de Mar del PlataMar del PlataArgentina

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